In a radio communication system, when setting up radio access of a mobile terminal to a cellular wireless system, the mobile terminal has to be synchronized with the cellular wireless system. To achieve synchronization, one of the procedures to be performed is to obtain the symbol timing synchronization. This is typically done by letting the base station transmit a synchronization signal. The receiver of this synchronization signal, i.e. a mobile terminal, can then detect the symbol timing by correlating the received signal with a replica of the transmitted signal, creating a correlation peak at the correct timing location of the synchronization signal. Due to the many multiplications involved in the correlation procedure, this procedure is regarded as very computationally expensive and it is desirable to reduce this effort.
Synchronization between a base station and a mobile terminal has to be performed in all types of radio communication systems and for all types of information carrying techniques, multi-carrier or single-carrier techniques, such as OFDM, FDMA, MC-CDMA and the like. In this description, the present disclosure will be exemplified in a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system using OFDM. However, as is clear to a skilled person, the present disclosure is not limited to this exemplification and may be utilised in essentially any radio communication system using any information carrying technique.
In 3GPP LTE, the Primary Synchronization Channel (P-SCH) signals are obtained from any of three different Zadoff-Chu (ZC) sequences. An OFDM signal is then generated from an Inverse Discrete Fourier Transform (IDFT) of a frequency domain defined ZC sequence, mapped on the used subcarriers, excluding the DC subcarrier. The receiver detects, by correlation with the three replica signals, which ZC sequence was used, and the timing synchronization of the signal.
In prior art, the ZC sequences is proposed to map on the subcarriers so that time domain centrally symmetric synchronization signals are produced. The receiver may then, when correlating the synchronization signals with a replica signal, utilize the centrally symmetric signal property to reduce the multiplication complexity by adding every pair of received centrally symmetric samples before multiplication with the corresponding value of the replica signal. By this, one multiplication per each pair of symmetric samples is avoided. Thus, a reduction of the computational complexity of about 50% can be achieved, compared to a conventional implementation of the correlation.
In addition, for odd-length ZC sequences which are mapped to provide centrally symmetric signals, it is possible to obtain two synchronization signals (from two different ZC sequences), which are complex conjugated versions of each other. Thereby, in the receiver, the detection of the two signals can be implemented with a multiplication complexity of just one of the signals. Hence, the centrally symmetric property of the synchronization signal is in a twofold way useful for implementing low-complex detection.
By the use of centrally symmetric signals, a reduction of the number of multiplications, obtained by adding symmetric samples, is only feasible when the whole synchronization signal is used in the correlation. Such a full-length correlation procedure is typically used when the frequency offsets are small. Small frequency offsets can be assumed for, for example, neighbor cell search.
On the other hand, when the frequency offsets are large, which can be assumed for, for example, initial cell search, the received signal becomes distorted. Therefore, a full-length correlation does not give a good result for large frequency offsets. For this case, the correlation has to be computed on smaller parts of the synchronization signal and the correlation values from the different parts are then non-coherently added.
When correlations are computed for parts of the synchronization signal, the complexity reduction may not be achieved as efficiently as for the full-length correlation of the centrally symmetric signal in the prior art solutions, since it is not sure that each of the samples within such a part of the signal will have a corresponding symmetric sample to be added together with before the multiplication. Thus, in the prior art solutions, there is a computation complexity problem present for correlations being performed on shorter parts than the whole length of the synchronization signal.